Current video coding methods employ a wide variety of techniques such as motion estimation, motion compensation, deblocking, and filtering for video compression. Conventional methods use a differential coding loop, wherein a video picture is broken down into blocks, undergoes prediction coding, and is then reconstructed and used as reference for a next picture undergoing prediction coding.
Recent developments in coding have utilized in-loop Wiener filters to further improve the quality of coded pictures. A Wiener filter works to reduce the error between an original signal and a noisy signal (a signal with certain errors inherent as a result of the coding process). Wiener filters first use an autocorrelation matrix and crosscorrelation vectors to estimate filter parameters. This estimation typically occurs after the deblocking stage. Please refer to FIG. 1. FIG. 1 is a diagram of a conventional video encoder including an adaptive loop filter in the differential coding loop. The video encoder 100 comprises a motion estimation/motion compensation (ME/MC) block 110; an intra prediction block 105; a macroblock mode decision block 155; a transform/quantization (T/Q) block 140; an inverse transform/inverse quantization (IT/IQ) block 145; a reconstruction unit 135; a deblocking unit 130; an entropy coding unit 150; a filter parameter estimator 125; an adaptive loop filter 120; and a reference picture buffer 115. Wiener-Hopf equations are formed and solved by the filter parameter estimator 125 to estimate the filter parameters. These equations are formed by accessing the original picture (from the input) and the deblocked picture (from the deblocking unit 130), for calculating the autocorrelation of the deblocked signal (i.e. a to-be-filtered signal) and the crosscorrelation between the original signal (from the input) and the deblocked signal, where the two accesses are done simultaneously. This is known as a first pass. The Wiener filter coefficients must then be applied to the deblocked picture, in order to generate the signal with smaller errors. This is known as the second pass. The generated signal must then be written into the reference picture buffer 115, which stores the reconstructed pictures for future reference. These processes are illustrated in FIG. 2. As can be seen from the diagram, the conventional two-pass process requires three read operations and a write operation per pixel. Frequent DRAM access is required for these processes and therefore the encoding latency is increased.
FIG. 11 is a diagram of a conventional video decoder including an adaptive loop filter in the differential coding loop. The video decoder 1100 comprises an MC block 1120; an intra prediction block 1115; an IT/IQ block 1110; a reconstruction unit 1135; a deblocking unit 1140; an entropy decoding unit 1105; an adaptive loop filter 1130; and a reference picture buffer 1125.
Wiener filtering can also be applied in the inter prediction stage. Please refer to FIG. 3. FIG. 3 is a diagram of a conventional video encoder including an adaptive interpolation filter in the differential coding loop. The video encoder 300 comprises an ME/MC block 310; an intra prediction block 305; a macroblock mode decision block 355; a T/Q block 340; an IT/IQ block 345; a reconstruction unit 335; a deblocking unit 330; an entropy coding unit 350; a filter parameter estimator 325; an adaptive interpolation filter 320; and a reference picture buffer 315. The filter parameter estimator calculates optimal filter parameters, and the adaptive interpolation filter reduces the error between an original signal and a prediction signal. For a current picture, the optimal filter parameters are initially unknown. The filter parameter estimator forms Wiener-Hopf equations by calculating the autocorrelation of the prediction signal (i.e. a to-be-filtered signal) and the crosscorrelation between the original signal (from the input) and the prediction signal (from macroblock mode decision output), where the prediction signal is obtained by interpolating reference pictures with predefined standard 6-tap filter coefficients. Then the filter parameter estimator solves the equations to obtain the optimal filter parameters. This is known as a first pass. Next, the reference pictures of the current picture are interpolated and filtered with the optimal filter parameters for ME/MC of the second time. This is known as the second pass. The second pass can be iteratively performed until convergence. These processes are illustrated in FIG. 4. As can be seen from the flowchart, the conventional multi-pass process requires processing ME/MC twice or more. The encoding latency and complexity are both significantly increased.
FIG. 12 is a diagram of a conventional video decoder including an adaptive interpolation filter in the differential coding loop. The video decoder 1200 comprises an MC block 1220; an intra prediction block 1215; an IT/IQ block 1210; a reconstruction unit 1235; a deblocking unit 1240; an entropy decoding unit 1205; an adaptive interpolation filter 1225; and a reference picture buffer 1230.